THE JOURNAL OF CHEMICAL PHYSICS 134, 024515 (2011)
Structural study of low concentration LiCl aqueous solutions in the liquid,
supercooled, and hyperquenched glassy states
K. Winkel,1 M. Seidl,1 T. Loerting,1 L. E. Bove,2 S. Imberti,3 V. Molinero,4 F. Bruni,5
R. Mancinelli,5 and M. A. Ricci5,a)
1
Institute of Physical Chemistry, University of Innsbruck, Innrain 52a, 6020 Innsbruck, Austria
IMPMC, Université P. et M. Curie et CNRS-UMR, 7590 Paris, France
3
STFC, ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus,
Didcot, Oxon OX11 0QX, United Kingdom
4
Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA
5
Dipartimento di Fisica “E. Amaldi,” Università degli Studi “Roma Tre,” Via della Vasca Navale 84,
00146 Roma, Italy
2
(Received 10 September 2010; accepted 24 November 2010; published online 12 January 2011)
Neutron diffraction experiments on a solution of LiCl in water (R = 40) at ambient conditions and in
the supercooled and hyperquenched states are reported and analyzed within the empirical potential
structure refinement framework. Evidence for the modifications of the microscopic structure of the
solvent in the presence of such a small amount of salt is found at all investigated thermodynamic
states. On the other hand, it is evident that the structure of the hyperquenched salty sample is similar
to that of pure low density amorphous water, although all the peaks of the radial distribution functions
are broader in the present case. Changes upon supercooling or hyperquenching of the ion’s hydration
shells and contacts are of limited size and evidence for segregation phenomena at these states does
not clearly show up, although the presence of water separated contacts between ion of the same sign
is intriguing. © 2011 American Institute of Physics. [doi:10.1063/1.3528000]
I. INTRODUCTION
Amorphous polymorphism (polyamorphism) is one of
the key concepts in our current understanding of water, its
structure, physical properties, and anomalies.1–5 The basic
idea is that liquid water at ambient conditions (average density = 1.00 g/cm3 ) is constantly fluctuating between highdensity domains and low-density domains.6 The high- and
low-density domains cannot be isolated at ambient conditions, but it may be possible to obtain phase segregation in the
deeply supercooled regime at temperatures below 235 K. Unfortunately, experiments on bulk water cannot be done in this
temperature region because of the homogeneous nucleation
limit and rapid crystallization, which prevents supercooling of
liquid water below 235 K (in the so-called “no-man’s land”).7
One way of avoiding rapid crystallization is by cooling water at rates which even exceed the crystallization rate (hyperquenching). Samples are typically cooled to 77 K, at cooling rates up to 107 K/s (Ref. 8) and a vitrified, glassy state
of matter is eventually obtained. Interestingly, hyperquenched
glassy water has a density of ∼0.92 g/cm3 , representing a lowdensity form of noncrystalline water, often also referred to as
low-density amorphous ice (LDA). Thus we could infer that
upon hyperquenching the low-density domain structure is preferred over the high-density domain structure. Devitrification,
i.e., the transition from the glass to the deeply supercooled liquid, was found to take place close to Tg ∼ 136 K; above this
temperature, a sudden increase in heat capacity,9 isotope mixing of layers,10 and sudden penetration of an indentor into the
a) Electronic mail: [email protected].
0021-9606/2011/134(2)/024515/8/$30.00
sample11 were reported. Unfortunately, these samples crystallize above approximately 150 K, leaving a rather narrow
window for experiments on deeply supercooled water, at ambient pressure. Upon applying pressure LDA transforms to the
high-density amorphous form of ice (HDA), with density of
1.15 g/cm3 at ambient pressure.12 This transition takes place
suddenly in a first orderlike manner at ∼700 MPa at 77 K
and ∼450 MPa at 125 K.13 Interestingly it has been argued
that, with increasing pressure, the microscopic structure of
supercooled water evolves from a low-density liquid, similar
to LDA, to a high-density phase, similar to HDA.14 While in
pure water the existence of the no-man’s land prevents a direct experimental answer of the question whether or not the
glassy states of water are thermodynamically continuously
connected to ambient water, the continuity can be shown in
aqueous solutions in the pressure range up to 200 MPa. In this
manner the study of aqueous solutions at low temperatures is
inevitably connected to polyamorphism.15
Introduction of a small amount of salt solutes has been
shown to affect the structure of water in a manner similar to
the application of high pressures.16–18 Conversely, introduction of a large amount of solute results in partial destruction
of the topological elements found in bulk water due to the
increasing number of incomplete hydration shells. Thus the
transition from dilute to concentrated solutions is typically put
at a ratio of solute:water molecules = 1:20 (R = 20). In the
case of LiCl, concentrated solutions (R ≤ 12) can easily be
vitrified by use of conventional cooling of rates up to 102 K/s,
without the need for hyperquenching techniques, while for dilute solutions of R ≥ 15 vitrification requires high rates and
hyperquenching.19, 20 In this work we choose to study dilute
134, 024515-1
© 2011 American Institute of Physics
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024515-2
Winkel et al.
aqueous solutions of LiCl at R = 40, for the reasons given below. A very interesting aspect is that the glass-to-liquid transition temperature Tg shows a highly nonlinear dependence
on solute concentration (see Fig. 3 in Ref. 20). Initial increase of solute concentration from R = ∞ (pure water) to
R = 40 results in a decrease of Tg from 136 to 129 K. Further increase from R = 40 to R = 15 results first in a slow
increase, followed by a sudden increase at R = 21 from 132
to 142 K. That is, there is a Tg -minimum at R = 40 and a
maximum at R = 18. Since materials at their Tg ’s are considered to be in an isoviscous state, a minimum in Tg implies a minimum in viscosity (as a function of composition
at fixed temperature).20 Thus in the case of LiCl solutions, at
R = 40 the maximally plasticized state of the water network
and the maximum of water fluidity is attained. It is conceivable that the occurrence of this state of maximum fluidity is
related to polyamorphism in aqueous solutions, and hence it
is important to investigate this state further. In addition to the
minimum in Tg also the increase in heat capacity at Tg (c p )
shows a minimum; as a consequence, the low-density configuration can be labeled as “strong glass former,” according to
the Angell’s scheme.21
Suzuki and Mishima have even shown that dilute glassy
solutions can be phase separated into two parts: a highly concentrated solution of LiCl, which may in a way resemble
HDA, and low-density glassy water,22 which resembles LDA.
This phase separation may go hand in hand with an increase
in ion-pairing, which was found earlier in the glassy state,23 or
with other clear changes of the microscopic structure of water.
In the case of LiCl the Tg minimum and the subsequent sharp
Tg rise are much more pronounced than in other chlorides
such as NaCl, CsCl, or MgCl2 (see Fig. 7 in Ref. 24). While
all lithium-halides show this type of behavior, the minimum
and the sharp rise are most pronounced in LiCl (see Fig. 6 in
Ref. 24). We, therefore, report here a structural study of LiCl
solutions (R = 40) in the glassy state, in the metastable (supercooled) liquid state, and in the stable liquid state, with the
major aim of comparing especially the water–water radial distribution functions (RDFs) with pure hyperquenched glassy
water (HGW) and furthermore to analyze ion–water and ion–
ion interaction for checking for increase in ion-pairing at low
temperatures. There is one thing that we want to emphasize at
the outset: future work at different solute:water ratios (e.g., at
R = 30, 18, and 10) will be necessary to learn more about the
structural aspects underlying the unexpected behavior of the
glass transition temperature as a function of R.
II. EXPERIMENT AND DATA REDUCTION
A. Sample preparation
The LiCl–water (–heavy water) solution was prepared
from Sigma anhydrous LiCl (99.98%) dissolved in deionized distilled water (heavy water) in the appropriate proportion (R = 40) under Ar atmosphere. The two mixtures have
been mixed under Ar atmosphere in the appropriate proportion in order to obtain a 50% H/D mixture. The solutions have
been filtered through a 0.2 μm nanoporous filter and stored
into cylindrical glass tubes flame sealed. The tubes were
kept in liquid nitrogen during sealing in order to avoid water
J. Chem. Phys. 134, 024515 (2011)
evaporation. The concentration of the three samples has
been checked by the “Service Central d’Analyse du CNRS
(SR-59).”
To form a glassy state of dilute LiCl aqueous solution, high cooling rates are necessary.20 Therefore the hyperquenching technique was used.8 The hyperquenched samples
were prepared as described in Ref. 25 for pure water (HGW).
Micrometer-sized droplets were made with an ultrasonic nebulizer operating at 3 MHz (LKB Instruments, model 108, with
the parameters 80% H2 O, 4 l/min, N2 carrier gas 2.0 l/min).
The aerosol was transferred with nitrogen (purity of 99.995%)
as carrier gas through a silicon tube which is cooled with ice
water in order to reduce the relative amount of water vapour.
The aerosol enters the vacuum chamber through an aperture
of 300 μm diameter and is deposited on a cryoplate (Cu) at
77 K. Deposition time varied between 70 and 80 min.
B. Neutron diffraction experiment
The experiment has been performed at the SANDALS
diffractometer,26 installed at the ISIS pulsed neutron source
(UK). Liquid samples were contained into standard Ti–Zr flat
cells of t = 1 mm internal thickness and 1 mm wall thickness, while cells with t = 2 mm were used for the HGW samples. Sample containers were connected to a He cryostat for
the temperature control. Measurements on the liquid samples
have been performed at T = 298 K (ambient) and T = 268 K
(supercooled), while the hyperquenched sample (HGW), after
storage and recovery in liquid nitrogen, has been measured at
T = 80 K. In order to apply the H/D contrast method in the
neutron diffraction experiment, water hydrogens have been
isotopically substituted (see Sec. II A) and measurements
have been performed at each temperature on three different
isotopic mixtures: i.e., one fully deuterated, one fully protonated, and an equimolar mixture of the two (hereafter labeled
HDO) in the case of liquid samples, while the deuterated sample was contaminated during the hyperquenching procedure
and its D/H ratio was 3:1. For each sample, data have been
accumulated for about 8–10 h, with ISIS running on average
at 180 μA; measurements have also been performed for the
empty containers, empty instrument, empty cryostat, and a
standard vanadium slab (5.0 × 5.0 × 0.348 cm3 ).
C. Data reduction
Data reduction has been performed by using the suite of
programs (GUDRUN) (Ref. 27) available for SANDALS and
widely described elsewhere.28 This performs corrections for
multiple scattering, absorption, and inelasticity effects, subtracts the sample container contribution, and normalizes the
data to an absolute scale. GUDRUN also performs a check of
the sample density, ρ, and composition against the high Q
limit of the total measured differential scattering intensity, defined as
ρt
cα σα
lim I (Q) =
Q→∞
α
4π
,
(1)
where Q is the modulus of the exchanged momentum and cα
and σα are the fraction and total scattering cross sections29
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024515-3
LiCl aqueous solutions
J. Chem. Phys. 134, 024515 (2011)
H2O
0.4
F(Q) [barn/(atom sr)]
F(Q) [barn/(atom sr)]
0.6
0.2
0
-0.2
D2O
-0.4
0.5
H2O
0
D2O
-0.5
HDO
0
5
15
10
20
-1
Q [Å ]
HDO
-1
0
2
4
6
8
-1
FIG. 1. Measured differential cross sections of three LiCl–water solutions
(R = 40) at ambient temperature (solid line), along with the residuals of the
EPSR fit (dashed lines). Data have been offset for clarity.
of atoms of type α. In the case of the hyperquenched
samples, this level is used to establish the fraction of void volume in the sample container, assuming for the sample a density of 0.0934 atoms/Å3 . This density value has been calculated from the density of pure HGW (0.094 atoms/Å3 ), taking
into account the ratio between the molar mass of the solution
and pure water, and double checked against the shift of the
first halo peak in the x-ray diffraction pattern (not shown) of
HGW–LiCl compared to amorphous ices with different densities.
The final output of the data reduction procedure is the
neutron differential cross section (DCS):
cα cβ bα bβ (2 − δαβ )(Sαβ (Q) − 1),
(2)
F(Q) =
α β>α
where bα is the neutron coherent scattering length29 of the α
species. Sαβ is the partial structure factor (PSF), namely, the
Fourier transform of the RDF of the α, β pair,
∞
sin(Qr )
dr. (3)
r 2 (gαβ (r ) − 1)
Sαβ (Q) = 1 + 4πρ
Qr
0
Since hydrogen and deuterium have markedly different scattering lengths, the H/D substitution on the solvent gives access to three independent DCS data sets, characterized by a
good contrast one to each other (see Fig. 1).
A LiCl–water solution consists of m = 4 different atomic
species (namely, H/D, O, Li, and Cl); consequently, each DCS
is the combination of m(m + 1)/2 = 10 distinct PSFs and
complete exploitation of the microscopic structure of this solution is subject to the inversion of a system of three equations with ten unknowns [see Eq. (2)], which are the individual PSFs. The solution of this inversion is not unique and
can only be performed with the support of a proper computer simulation model and based on complementary knowledge, such as for instance molecular geometry of water, presence of H-bonds between water molecules, and ionic charges.
In the present instance we have used the empirical potential
structure refinement (EPSR) (Ref. 30–32) code. Applications
of this code to studies of aqueous solutions are thoroughly
described in Refs. 16, 17, 28, and 33–40. Briefly, this is a
standard Monte Carlo simulation of the solution in question,
Q [Å ]
FIG. 2. The measured differential cross sections of three isotopically substituted LiCl aqueous solutions (R = 40) at different temperatures, namely, at
ambient temperature (black line), in the supercooled liquid state (red line),
and in the HGW state (blue line). The vertical dashed lines indicate the position of the first diffraction peak of pure D2 O and pure HDO at ambient
conditions (Ref. 45) (black dashed); the blue dashed lines indicate the position of the same peaks in a pure HGW sample (Ref. 46). Data have been
offset for clarity.
which iteratively adjusts a perturbation to the starting reference potential until the simulated DCS converges to a satisfactory fit of the measured data. This is usually achieved after
hundreds of iterations, provided that the starting reference potential can mimic the relevant features of the system, such as
the presence of a network of H-bonds in the present instance.
Several different choices of this potential model may be reasonable, and many are available in the literature at least for
water; nevertheless, the experience over the years has demonstrated that the different choices do not dramatically affect the
final result, when used within the EPSR code.41, 42 In particular the range of RDF compatible with the experimental data
is usually rather narrow, although the degree of reliability of
the individual RDF depends, of course, on its weight within
the DCS functions. Specifically at low solute concentrations,
water–water RDFs will have the higher weight, followed by
solute–water ones, while solute–solute terms will give the
lower contribution to the measured DCS. After convergence
to a good fit, the EPSR simulation proceeds by accumulating
molecular configurations compatible with the data. These are
finally used to evaluate the whole set of site–site RDF, along
with other structural properties of interest, as. for instance.
the distribution of H-bonds. Consequently the DCSs reported
in Figs. 1 and 2 are experimental data, while the RDFs discussed in the following sections are the results of the EPSR
simulations. A detailed discussion about their uniqueness and
comparison with other inversion techniques, such as reverse
Monte Carlo, can be found in Refs. 30–32, 41, and 42.
For the present solutions, we have used as reference
potential the simple point charge/extended model43 for
water–water interactions and Lennard-Jones plus fractional
charges for the solutes,44 according to Table I. The Lorentz
Berthelot rules have been applied for the interaction between
different species. The simulation box dimensions and composition are reported in Table I for the liquid and hyperquenched
(HGW) samples.
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Winkel et al.
J. Chem. Phys. 134, 024515 (2011)
TABLE I. Lennard-Jones parameters and effective atomic charges used to
start the EPSR refinement of the neutron diffraction data, taken from Ref. 43
and Refs. 36 and 44 for water and LiCl, respectively. Sample density and size
of the EPSR simulation box, along with the number of water molecules, Li+ ,
and Cl− ions in the box, are reported in the bottom part of the table. HGW
labels the hyperquenched sample.
Sample
liquid
HGW
(kJ/mol)
0.65
0.00
0.69
0.57
σ (Å)
3.166
0.000
1.500
4.200
q(e)
−0.8476
0.4238
1.0000
−1.0000
density
(atoms/Å3 )
0.0992
0.0934
L box
(Å)
26.42
26.96
H2 O
Li+
Cl−
600
600
15
15
15
15
B. Water–water radial distribution functions
The quality of the fits obtained for the present data is
shown in Fig. 1, where the DCS functions for ambient temperature samples are reported along with the residuals of the
corresponding EPSR simulation. The fits at the other thermodynamic states are of the same quality.
III. RESULTS AND DISCUSSION
A. Differential cross sections
It is instructive to comment on the behavior with temperature of the DCS data, before looking and commenting EPSR
results in r -space. These data (see Fig. 2) show indeed clear
trends with temperature and differences with the case of pure
water,45, 46 which are worth to be described. The DCS of pure
H2 O is characterized by a double peak in the range 3–6 Å−1 :
the position of the two maxima does not sensibly change going from ambient to the HGW state, while their relative intensity changes. The same behavior is found in the case of the
LiCl solution, with the intensity of the first peak increasing
in the supercooled and in the HGW states. The HDO mixture
in pure ambient water has an asymmetric first peak at ∼2.06
Å−1 , which moves to smaller Q-values on lowering the temperature and goes to 1.71 Å−1 in the HGW state. In our sample this peak comes at Q = 2.11 Å−1 at ambient temperature, moves to ∼2 Å−1 in the supercooled state, and reaches
Q = 1.83 Å−1 in the HGW sample. Interestingly both our
HGW data and those of Ref. 46 show that the first peak of the
DCS sharpens and develops a secondary structure at 2.8–2.9
Å−1 in the glassy state. As far as the D2 O data are concerned,
in pure water the first peak (at 1.98 Å−1 at ambient conditions) moves to smaller Q-values upon supercooling and goes
to 1.7 Å−1 in the HGW sample. We notice the same trend in
the LiCl solution, with the peak at 2.02 Å−1 at ambient temperature, moving to 1.92 Å−1 in the supercooled state. Our
HGW sample is, as already noticed, contaminated by H2 O;
thus, the comparison with pure deuterated HGW is not completely appropriate; the misfit with DCS data of the other two
samples at Q-values larger than 4 Å−1 is also due to contamination. We stress that contamination does not however spoil
our simulation results, since the simulation box of the HGW
sample has been built using the real sample composition.
In pure ambient water the almost tetrahedral network of
H-bonds is manifested by the position of the first two peaks
of the gOO (r ) at 2.8 and 4.5 Å, respectively. Solvation of ions
is known to affect the position of the second peak, while leaving almost unperturbed the first one,34 in a manner similar to
the application of an external pressure. The present LiCl solution confirms this finding (see Fig. 3), with the second peak
at 3.9 Å, corresponding to an equivalent pressure of 307 MPa.
On lowering the temperature the first peak moves to shorter
distances (this is best visible in the HGW sample), as found
also in pure water,47 while the second one goes to larger distances. This suggests that at lower temperatures the network
tends to recover tetrahedrality, although the second neighbor
shell in our HGW sample is closer to the first one compared
to pure HGW (Ref. 46) (see Fig. 4). We notice also that the
salty HGW looks less ordered than the pure water one, with
less intense and broader peaks of all three RDFs.
When pure water is supercooled, the gHH (r ) shows very
little changes,47 on the contrary in this solution we notice that
both peak position and intensity change on supercooling (see
Fig. 3), with the two main peaks going far apart in the HGW
6
a
10
OO
8
b
5
4
3
g(r)
Atom
O
H
Li
Cl
In summary, the DCS data of the LiCl aqueous solutions
show the same temperature behaviors as pure water ones, as
far as shift and peak intensity are concerned. Nevertheless the
first peaks of HDO and D2 O samples at ambient conditions
are at larger Q-values than in pure water and do not reach
the same positions when the sample is hyperquenched. Since
the DCS signals at the present solute concentration are dominated by the water contribution, these findings are the signature of differences in the microscopic arrangement of water
molecules in the solution compared to neat water: possibly the
same differences which depress the crystal nucleation temperature of the solution.
g(r)
024515-4
OH
4
2
HH
0
2
4
r [Å]
6
OH
2
1
0
OO
6
8
0
HH
0
2
4
r [Å]
6
8
FIG. 3. The EPSR calculated radial distribution functions of water atoms in
LiCl (R = 40) solution (a) and in pure water (b). As in Fig. 2, data at ambient
conditions are reported in black, those in the supercooled state in red, and
finally the HGW data in blue. Data for OO and OH pairs have been offset for
clarity. Note that in the case of pure ambient water also, the intramolecular
peaks have been reported. Data for pure water are from Ref. 31, 46, and 47,
respectively.
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024515-5
LiCl aqueous solutions
J. Chem. Phys. 134, 024515 (2011)
10
HB distribution
OO
6
4
OH
0.3
0.2
0.1
2
HH
0
2
4
r [Å]
6
0
8
FIG. 4. The EPSR calculated radial distribution functions of water atoms in
the hyperquenched glassy state, in the LiCl solution (blue), and in pure water
(black) (Ref. 46). Data for OO and OH pairs have been offset for clarity.
sample. The peak positions for this sample are the same as
for pure HGW (Ref. 46) (see Fig. 4), but the first peak is less
intense and broader.
In pure water supercooling is associated with the sharpening of the first peak of the gOH (r ),47 that is the signature of increased directionality of the H-bonds. In the salty
sample instead both first and second peaks become sharper,
and additionally their intensities increase (see Fig. 3). Moreover the first peak moves to shorter distances with hyperquenching, suggesting a shortening of the H-bonds, while
the intensity of the second peak decreases. The position
of the first peak is the same as for pure HGW (Ref. 46)
(see Fig. 4), although the peak width is almost twice and
the second neighbor shell is centered at larger distances.
We notice also that the second peak is overall broader and
does not show the shoulder at ∼3.74 Å, which characterizes
pure HGW.
Due to the above reported changes of the RDF, the average number of neighboring oxygens around a water molecule
decreases with temperature from 4.3 ± 1.1 at ambient conditions to 4.0 ± 1.0 in the supercooled state and 3.70 ± 0.91 in
the HGW sample,48 while the average number of H-bonds,
as calculated from the distribution functions reported in
Fig. 5, increases from 3.0 to 3.2–3.3 in the supercooled and
HGW samples.
All above findings are consistent with an increase of the
H-bond strength and average number, with consequent reduction of non-H-bonded molecules within the mesh of the
H-bond network (the so-called interstitial molecules), on going from ambient to the supercooled and hyperquenched
states. In particular the peak positions of the RDFs of the
HGW sample are identical to those of the pure HGW, although the overall broadness of the peaks in the salty sample suggests that the presence of salts determines a higher
degree of disorder in the structure. Overall, the radial distribution functions of salty water are broader than those of neat
water, but their temperature behavior is similar.
0
1 2 3 4 5 6
nHB
FIG. 5. Distribution function of the hydrogen bonds at the three investigated state points. Two water molecules are considered H-bonded when the
H site of the second molecule sticks by the oxygen of the first one within
a distance shorter than or equal to the position of the first minimum of the
gOH (r ).
C. Li + hydration shell
The Li+ hydration shell is well defined and stable with
temperature, with average distances of 1.95 and 2.64 Å for the
Li–O and Li–H pairs (see Fig. 6). These distances are compatible with a close alignment of the O–Li director with the water dipole, as already found in the case of aqueous solutions
of LiOH.36 We notice however that in the present case the
Li–O distance is shorter than it was in the hydroxide solution;
this difference might be due to the lower solute concentration
of the present sample, as suggested by the weak concentration dependence of the cation–oxygen distance evidenced in
Ref. 36. We notice also that ab initio simulations of a Li+ ion
in a box with 32 water molecules49 predict a Li–O distance
even shorter than that found here. The number of water oxygens in the Li+ hydration shell is of the order of 4 and shows a
weak temperature dependence (3.91 ± 0.49, 3.80 ± 0.40, and
3.79 ± 0.45 for ambient, supercooled, and HGW samples,
15
g(r)
g(r)
8
0
ambient
supercooled
HGW
0.4
10
LiO
5
LiH
0
0
2
4
r [Å]
6
8
FIG. 6. The EPSR calculated radial distribution functions of water atoms
around a Li+ ion (same colors as in Fig. 2). Data have been offset for clarity.
The first sharp peaks of the two radial distribution functions define the Li+
hydration shell, formed by about four water molecules.
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024515-6
Winkel et al.
J. Chem. Phys. 134, 024515 (2011)
7
25
6
ambient
supercooled
HGW
20
4
gLiCl(r)
g(r)
5
ClO
3
15
10
2
5
1
ClH
0
0
2
4
r [Å]
6
0
8
FIG. 7. The EPSR calculated radial distribution functions of water atoms
around a Cl− ion (same colors as in Fig. 2). Data have been offset for clarity.
The first intense peaks of the two radial distribution functions define the Cl−
hydration shell, formed by about six water molecules.
respectively), which however has no statistical relevance and
cannot be considered as a signature of phase separation.
D. Cl− hydration shell
Each Cl− ion accommodates about six water hydrogens
in its hydration shell (precisely 6.0 ± 1.3, 6.0 ± 1.2, and 5.5
± 1.3 for ambient, supercooled, and HGW samples, respectively) and between eight and seven oxygens. Although these
numbers show a trend with temperature, their statistical uncertainty does not allow to claim about phase separation. The
Cl–H and Cl–O shells (see Fig. 7) are centered at 2.18 and
3.15 Å at ambient conditions, i.e., at slightly shorter distances
compared with the findings of Jal et al.50 at higher salt concentration, by first difference neutron diffraction.
The present data confirm that the hydration shell of
Cl− in salt solutions is shorter than that found in acidic
solutions.17, 39 Interestingly, for the HGW sample both Cl–
H and Cl–O distances increase, allowing a better Cl–H–O
alignment.
0
2
4
r [Å]
6
8
FIG. 8. EPSR calculated radial distribution function of the Li+ –Cl− pair
(same colors as in Fig. 2). In spite of the high intensity of the first peak, the
number of ion–ion contacts is negligible at this concentration at all temperatures.
and HGW phases. It is not clear however if this phenomenon
may be the signature of the onset, by lowering the temperature, of the phase separation predicted at higher pressures.51, 52
IV. CONCLUSIONS AND OUTLOOK
We have studied the structure of dilute aqueous solutions
of LiCl using neutron diffraction experiments at ambient conditions (298 K), in the supercooled state (268 K), and in the
hyperquenched glassy state (80 K). A ratio LiCl:H2 O = 1:40
(R = 40) was employed because at this dilution level LiCl
solutions are known to be of the highest fluidity at 130 K.
Interestingly, the glass-to-liquid transition temperature Tg at
this dilution is lower by a few Kelvin compared even to pure
water.20 By contrast, the state of the lowest fluidity showing
a Tg by a few Kelvin higher compared to that of pure water is found at a ratio of R = 18, which is subject to future
investigation. It may be possible that this unusual behavior of
E. Ion contacts
3
g(r)
The gLiCl (r ), reported in Fig. 8, shows a very sharp, intense peak at 2.37 Å due to Li–Cl contacts, followed by a
second peak at ∼4.7 Å, which corresponds to ∼1 water separated Li–Cl
pair. The number of direct contacts, calculated
as n(r ) = 4πρr 2 gLiCl (r ) dr over the first peak, is between
0.2 and 0.3 per ion, with an uncertainty of ±0.4, meaning that
recombination is a rare event at this solute concentration and
thermodynamic states.
The gLiLi (r ) and gClCl (r ) functions (see Fig. 9) show an
interesting behavior with temperature. In particular, gClCl (r )
shows two neighboring shells at ∼5 and ∼7 Å and only in
the HGW sample the presence of a prepeak at ∼4 Å due
to water separated contacts. The presence of water separated
pairs is evident at all temperatures in the gLiLi (r ) function,
where a small prepeak at ∼3.7 Å is already visible at ambient
conditions and becomes an intense peak in the supercooled
Li Li
2
1
Cl Cl
0
2
4
6
r [Å]
8
10
FIG. 9. EPSR calculated cation–cation and anion–anion radial distribution
functions (same colors as in Fig. 2), showing the presence of water separated
contacts at about 4 Å in the HGW sample and in the case of Li+ also in the
supercooled liquid. Data have been offset for clarity.
Author complimentary copy. Redistribution subject to AIP license or copyright, see http://jcp.aip.org/jcp/copyright.jsp
024515-7
LiCl aqueous solutions
a maximum and minimum in Tg is related to the occurrence
of a changing fraction of LDA and HDA like configurations.
Upon lowering the temperature, the differential cross sections of LiCl aqueous solutions at R = 40 show the same
trends as pure water, but the radial distribution functions of
water, as for other salt solutions, are distorted with respect to
those of the neat solvent. In particular, at ambient conditions,
ions solvation determines a shrinkage of the second neighbor’s shell and a consequent loss of tetrahedrality, and the
HGW phase can be associated with the LDA state of water,
but not the HDA state. Indeed on lowering the temperature
through the supercooled phase to the HGW one, we notice
that the H-bonds recover linearity and the network tetrahedrality increases. The structure of the salty HGW is similar
to that of the pure HGW, although all the peaks of the radial
distribution functions are broader, suggesting that the distortions induced by the salt on the structure of water persist in
this state and determine a variety of different local configurations within the H-bond network. The number of neighboring water molecules decreases upon cooling and in the
HGW phase, thus confirming that this amorphous form of
the solution can be associated with the LDA polymorph of
water.
Changes of the hydration shell of the ions are little and
there is also little or no evidence for phase separation. Indeed the number of direct or water separated ion contacts does
not show a statistically relevant increase in the HGW phase,
although a peak ascribable to water separated cation–cation
and anion–anion contacts shows up in the HGW sample. That
ions of the same sign are brought closer to each other is an
intriguing issue, which merits further investigation from both
experimental and computational sides.
We want to emphasize that separation into two distinct
glasses, and a double-glass transition phenomenon has been
observed in LiCl solutions vitrified at high pressures,51, 52
by pressure-induced amorphization53 or pressure-induced
vitrification.54 The water-rich phase resembles the HDA
phase of bulk water,52 and, therefore, we plan to investigate
the difference between the low-pressure forms to the highpressure forms of the R = 40 solution in the future. This
HDA phase can be converted to the LDA phase in LiCl
solutions of R = 20 by decompressing the vitrified highdensity state at 142 K to 0.1 GPa.55 This decompressioninduced HDA→LDA transition is highly similar to the phasetransitions incurred upon decompressing bulk HDA, without
any salt, at similar temperatures.56, 57 The role of annealing on
the phase separation is another issue which deserves consideration in future experiments.
ACKNOWLEDGMENTS
The experiments on SANDALS have been performed
within the agreement between CCLRC and CNR, concerning collaboration in scientific research at the spallation neutron source ISIS and with partial financial support of CNR.
We would like to thank D. T. Bowron for providing the data
on HGW water, reported in Figs. 3 and 4 and A. P. Russo of
the Dipartimento di Fisica “E. Amaldi” (Università degli studi
Roma Tre) for computer support. We are grateful for finan-
J. Chem. Phys. 134, 024515 (2011)
cial support from the European Research Council (T.L., Starting Grant SULIWA), Austrian Academy of Sciences (M.S.,
DOC-fellowship), Austrian Science Fund FWF (K.W., Hertha
Firnberg position), “Italien Zentrum” of the University of
Innsbruck (K.W.), and Young Investigators Program of the
Beckman Foundation (V.M.).
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